1. Field of the Invention
The present invention relates to a hydraulic servo which operates a brake in forming a power transmission path in an automatic transmission of a vehicle, and to an automatic transmission incorporating such a hydraulic servo. More specifically, the present invention is directed to a hydraulic servo for a brake in an automatic transmission with provision for preventing rotation of its piston.
2. Description of the Related Art
Generally, a hydraulic servo for operating a brake in a vehicle automatic transmission comprises a piston member slidably mounted in a cylinder unit, a return plate fixed to the cylinder unit, and coil-shaped return springs compressed between the piston member and the return plate. Typically, the cylinder unit is formed within a transmission case, and an operating oil chamber is formed between the cylinder unit and the piston member which is sealed within the cylinder by various sealing members.
In such a hydraulic servo, responsive to supply of operating oil from a hydraulic control device to the operating oil chamber, the piston member is driven against the biasing force of the return springs toward the return plate, and an edge portion of the piston member comes into contact with and presses together the multiple friction plates forming a brake and disposed on an inner peripheral surface of the transmission case, for engagement of the brake. Responsive to discharge of the operating oil from the operating oil chamber, the piston member is axially moved back toward the bottom of the cylinder unit by the biasing force of the return springs, and the brake is thereby released.
In the conventional brake hydraulic servo mentioned above, because lubricating oil pools in the lower portion of the piston member when the brake is engaged, a drain hole is provided for draining the lubricating oil from that lower portion of the piston member.
Further, in the above described conventional transmission, in engaging a brake, the piston member presses against an axially outermost one of the friction plates which are spline-engaged with the transmission case. At this time, the outermost friction plate rotates only by the amount of play between the engaging splines, in the direction of rotation of inner friction plates which have been rotating prior to engagement, and simultaneously, the piston member pressing the outermost friction plate also rotates. One braking action causes only a slight rotation of the piston, but the piston member does not rotate in reverse when the brake is released, and therefore as the engagement and release of the brake is repeated numerous times, the piston member itself is gradually rotated. Accordingly, the position of the drain hole provided in what was initially the lower portion of the piston member rotates away from that lower position, resulting in lubricating oil pooling in the lower portion of the piston member when the brake is engaged.
To prevent the piston member from rotating, when the braking hydraulic servo is operated and the piston member engages the friction plates, a claw or a foot has been provided on the piston member, which claw or foot is received in a groove in the cylinder unit (Japanese Unexamined Patent Application Publication No. 2002-70753).
However, a protrusion such as a claw or a foot on a piston member may present problems in casting. For example, poor casting may result if the molten metal has not properly flowed (distributed) within the mold. On the other hand, if the protrusion is formed as a separate member and then joined to the piston, for example, affixing the claw or the like by welding or by using bolts becomes necessary, and this increases the number of parts and complicates the manufacturing process.